People who choose not to eat for 12 hours a day, aka those who fast, claim it gives you better sleep and abs. Are these people just annoying, or are they on to something?
Generally, intermittent fasting is a diet strategy that involves alternating periods of eating and extended fasting (meaning no food at all or very low calorie consumption).
“There’s quite a bit of debate in our research community: How much of the benefits of intermittent fasting are just due to the fact that it helps people eat less? Could you get the same benefits by just cutting your calories by the same amount?” said Courtney Peterson, an assistant professor in the Department of Nutrition Sciences at the University of Alabama at Birmingham who studies time-restricted feeding, a form of intermittent fasting.
We asked Peterson and a few other experts to help us sort out the real from the scam on intermittent fasting.
Q: How do I intermittent fast?
A: There are four popular fasting approaches: periodic fasting, time-restricted feeding, alternate day fasting and the 5:2 diet. Time-restricted feeding, sometimes called daily intermittent fasting, is perhaps the easiest and most popular fasting method. Daily intermittent fasters restrict eating to certain time periods each day — say, 11 in the morning to 7 at night. The fasting period is usually around 12 or more hours that, helpfully, includes time spent sleeping overnight. Periodic fasting will feel most familiar: No food or drinks with calories for 24-hour periods. Another type of fast, alternate day fasting, requires severe calorie reduction every other day. Lastly, the 5:2 method was popularized by author Kate Harrison’s book “The 5:2 Diet” and requires fasting on two nonconsecutive days a week.
Q: Is fasting an effective weight loss method?
A: If you are obese or overweight, fasting is an effective weight loss method, if you stick to it. But it is no more effective than a diet that restricts your daily calories. We know this because there were no additional weight loss or cardiovascular benefits of fasting two days per week over an ordinary calorie restriction diet in a study of 150 obese adults over the course of 50 weeks.
But you should also consider how difficult the diet will be to stick to. In a study of 100 randomized obese and overweight adults published in 2017, the dropout rate was higher with those who were fasting, 38%, compared to 29% for calorie restrictors and 26% for those who kept eating as they normally did.
“Some people really struggle with having to monitor their intake and constantly record food in an app every day. So the takeaway of the study was, if daily calorie restriction doesn’t work for you, maybe alternate day fasting would be a little easier,” said Krista Varady, professor of nutrition at the University of Illinois at Chicago and senior author of the study. “There’s nothing magical here. We’re tricking people into eating less food in different ways,” she said in 2017.
There is some new evidence that shows different forms of fasting are not equal — in part because some are easier than others but also because some forms of fasting better match our body’s natural circadian rhythm, thus lowering insulin levels, increasing fat-burning hormones and decreasing appetite.
Basically, because our metabolism has evolved to digest food during the day and rest at night, changing the timing of meals to earlier in the day may be beneficial.
In a study done in Peterson’s lab, 11 adults did time-restricted feeding (eating from 8 a.m. to 2 p.m.) and a control 12-hour eating period for four days each. On the last day of each session, researchers measured energy expenditure and hunger hormones and found that time-restricted feeding improves the appetite hormone ghrelin and increases fat burning. “It’s shown to reduce the amount of fat in the liver, which is a risk factor for diabetes and cardiovascular disease,” said Peterson.
Bottom line: If you want to lose weight and are someone who hates counting calories, you might consider fasting, as both methods offer similar weight loss benefits.
Q: Should I try intermittent fasting?
A: The most effective diet is the one you can stick to while still living your best life. It’s hard to know which will work best before trying, but doctors and recent studies offer some guidance. Peterson said that complete, zero-calorie fasts generally prove to be too difficult to maintain. “People stick with them maybe for the short-term, but they get quite hungry in the long-term,” she said.
Time-restricted feeding — fasting overnight and into the next morning — is likely the easiest form of fasting to comply with. A longer than normal fasting period each night allows you to burn through some of your sugar stores, called glycogen. That does a couple things. It gives your body a little bit more time to burn fat. It also may help your body get rid of any extra salt in your diet, which would lower your blood pressure, Peterson said.
Q: I’ve made the decision to fast for myself. So how long should I fast for?
A: There aren’t any studies right now that state exactly how long one should fast. Researchers, like Peterson, are working on that. The minimum amount of time it takes to make fasting efficacious hasn’t been proven via study, but the prevailing notion is it’s somewhere between 12 and 18 hours. But it can take a few days — sometimes weeks — of fasting regularly for your body to start burning fat for fuel. Brooke Alpert, nutritionist and author of “The Diet Detox,” suggests starting by moving your last meal to around 7 p.m. She said the reason for this is our bodies are better at doing some things at certain times. “Our bodies are better at processing sugar in the morning than at night,” said Varady. So eat bigger meals in the morning, for example.
And how often do you have to do daily intermittent fasting to see the benefit? Again, there hasn’t been a study that’s shown exactly how many days you need to fast, but a recent study in rodents showed they get about the same benefits fasting five days per week as they did fasting every day. “The great thing is that we’re learning that this type of fasting isn’t all or nothing,” Peterson said.